The present invention relates to cemented carbide bodies useful in tools for rock drilling, mineral cutting, oil drilling and in tools for concrete and asphalt milling.
In U.S. Pat. No. 4,743,515, cemented carbide buttons are disclosed having a core with finely and evenly distributed .eta.-phase embedded in the normal .alpha.+.beta.-phase structure, and a surrounding surface zone with only .alpha.+.beta.-phase (.alpha.=tungsten carbide, .beta.=binder phase, e.g., cobalt, and .eta.=M.sub.6 C, M.sub.12 C and other carbides, e.g., Co.sub.3 W.sub.3 C). An additional condition is that in the inner part of the surface zone situated close to the core, the cobalt content is higher than the nominal content of cobalt and that the cobalt content in the outermost part of the surface zone is lower than the nominal and increases in the direction towards the core up to a maximum, usually at the .eta.-phase core.
U.S. Pat. No. 5,286,549 discloses an improvement of the above-mentioned U.S. patent in which the cobalt content is essentially constant in the outer surface zone resulting in further increased wear properties.
According to U.S. Pat. No. 5,413,869, it has been found that further improvement is obtained in certain rock drilling applications if the core containing .eta.-phase is exposed on the top surface.
Cemented carbide bodies according to the above-mentioned patents are manufactured according to powder metallurgical methods: milling, pressing and sintering. The milling operation is an intensive mechanical milling in mills of different sizes and with the aid of milling bodies. The milling time is on the order of several hours up to days. Such processing is believed to be necessary in order to obtain a uniform distribution of the binder phase in the milled mixture, but it results in a wide WC grain size distribution.
In U.S Pat. Nos. 5,505,902 and 5,529,804, methods of making cemented carbide are disclosed according to which the milling is essentially excluded. In order to obtain a uniform distribution of the binder phase in the powder mixture, the hard constituent grains are instead precoated with the binder phase, the mixture is further mixed with a pressing agent, pressed and sintered. In the first mentioned patent, the coating is made by a SOL-GEL method and in the second, a polyol is used.
An important restriction of the above-mentioned prior art patents is the toughness properties of the cobalt rich zone. During the heat treatment process after sintering, the .eta.-phase in that zone is transformed to WC--Co resulting in a structure with both fine and coarse WC grains. Fine WC grain size in a cobalt rich matrix gives low resistance against plastic deformation in all applications where high forces and high temperatures are present such as in rock and coal cutting and hot forming. In these types of applications, there is substantial risk for damage of the whole tool caused by plastic deformation.
Another disadvantage of the prior art structure is the presence of both fine and coarse WC grains in the cobalt rich zone and the .eta.-phase core, leading to low resistance against crack propagation.